200 µl of a 154 mM NaCl solution containing 4 µCi ³⁶Cl was injected intraperitoneally. The isotope was allowed to equilibrate in the water space of the animal for 90 min (like in the experiments with isolated EDL). 15 min before the end of this period, the rats were anaesthetized by intraperitoneal injection of analgesics as described above. A blood sample was withdrawn from a tail vein for measurement of the content of ³⁶Cl in plasma. At the end of the equilibration period, EDL was exposed and stimulated at resting length via platinum wire electrodes at the indicated frequency and duration using 1-ms pulses at 10 V. Immediately after the stimulation, the muscles were washed with ice-cold Na⁺-free Tris-sucrose buffer, cut free, and washed four times for 5 min in ice-cold Na⁺-free Tris-sucrose, blotted, weighed, and soaked in 0.3 M TCA, and the extract was taken for the counting of ³⁶Cl. The uptake of ³⁶Cl is given as µmol/g wet wt ± SEM and intracellular ³⁶Cl as mM ± SEM, with the number of muscles in parentheses. All values were corrected for the loss of ³⁶Cl taking place during the washout in the cold.